This invention relates generally to anti-backlash gears, and more particularly to an anti-backlash twin gear composed of a pair of identical half-gears, no additional components such as springs being required.
Backlash refers to the lost motion between two intermeshed gears due to imperfect meshing of the gear teeth. Backlash is therefore the amount a drive gear must move before it transmits its motion to the gear driven thereby. In certain practical situations, the existence of backlash in a gear train transmitting power or motion cannot be tolerated.
For example, in an electronic timepiece, electrical timing pulses are generated which serve to actuate a stepping motor whose shaft is operatively coupled through a gear train to time-indicating hands. Should backlash exist between the drive pinion and the minutes wheel in the train, this permits the minute hand to float. As a consequence, the minute indication may be off by as much as one to three minutes.
Since a modern crystal-controlled electronic timepiece is inherently capable of providing highly accurate time indications, a one or two minute error by reason of mechanical backlash is not acceptable. Similarly, in servomechanisms and in variable speed drive mechanisms in which motion is transmitted through gears, the existence of backlash is often objectionable.
In order to avoid backlash, it is known to make use of anti-backlash gears of the double or split type constituted by two gear wheels of the same diameter and pitch, referred to as half-gears. One such half-gear is secured to a driven shaft while the other is free-running thereon, the two half gears being spring coupled to each other. In this arrangement, the bias developed by the spring normally acts to angularly displace the two half-gears so as to exert pressure on opposite tooth flanks of a driving pinion adapted to engage both half gears, thereby precluding lost motion.
The pre-loading of half-gears by compression or extension springs has a number of disadvantages; for the uniformity of the springs, their loading and the degree of displacement are very difficult to control. Moreover, gears pre-loaded by helical springs are generally restricted to relatively broad gears, whereas in many applications, such as in watches and other small instruments, the need exists for extremely thin anti-backlash gears.
Another practical drawback of anti-backlash gears of the type heretofore known, apart from the fact that such gears entail springs, restraining pins and other separate components which make assembly procedures complex and costly, is that the two half-gears are not identical, but are constituted by right- and left-hand units. These units must be separately fabricated and stocked, further complicating assembly procedures and adding to the expense of producing an anti-backlash gear.
Another limitation of conventional types of anti-backlash gears of the double type is that the displacement torque developed by the springs is relatively high, thereby introducing friction to the gears in mesh and requiring additional torque to drive the gear train. In the context of a battery-powered electronic timepiece, this requirement is a serious disadvantage, for it increases the energy demand and is reflected in shortened battery life.
Among the patents which disclose diverse forms of spring-biased twin gear arrangements adapted to minimize backlash are the following:
O'Neill U.S. Pat. No. 3,127,784 PA1 Finch U.S. Pat. No. 3,385,126 PA1 Thornburg U.S. Pat. No. 1,539,149 PA1 Fischer U.S. Pat. No. 3,407,727 PA1 Bethelsen U.S. Pat. No. 2,206,831 PA1 Winzeler et al. U.S. Pat. No. 3,535,948 PA1 Seabeck U.S. Pat. No. 3,296,881 PA1 Burke U.S. Pat. No. 1,197,261 PA1 Bliss et al. U.S. Pat. No. 2,928,288 PA1 Larson et al. U.S. Pat. No. 3,357,273